H1STOGENESIS AND STRUCTURE 



105 



portion of the J disk between the Z line and the succeeding Q disk. 

 It is interpreted more or less tentatively by Heidenhain as due to a linear 

 arrangement of J granules. The median segment of the J disk, that is, 



the portion between the N line and 

 the Z line, is called the terminal 

 disk of Merkel (E disk; endscheibe). 

 This complex condition of striping is 

 conspicuous only in certain insect 

 muscles (e.g., leg and wing muscles 

 of wasp, etc.) where powerful activity 

 or great rapidity of beat is required. 

 In general, complexity of striation of 

 conspicuous character corresponds 

 with a relatively greater irritability 

 and a capacity for more powerful or 

 relatively more rapid function. 



Another difference between skel- 

 etal and cardiac muscle pertains to 

 the diameter of the fiber and the posi- 

 tion of the nucleus. In skeletal mus- 





FIG. 115. SUCCESSIVE STAGES OF 

 SKELETAL MUSCLE HISTOGENESIS 

 IN MAMMALS. 



a, myoblast with fine cytoplasmic 

 granules (myochondria), from a 13 

 mm. sheep embryo; 6, myoblast 

 with homogeneous myofibrils, from 

 a 10 mm. guinea-pig embryo; c, 

 myoblast with cross striped fibrils, 

 from a 8.5 mm. rabbit embryo. 

 (From Heidenhain, after Godlew- 

 ski.) 



FIG. 116. TRANSVERSE 

 SECTION OF A STRIPED 

 MUSCLE FIBER OF A 

 NEWLY-HATCHED RAIN- 

 BOW TROUT, SHOWING 

 THE PROCESS OF MYO- 

 FIBRIL INCREASE BY RA- 

 DIAL LONGITUDINAL 

 SPLITTING. 



Mitochondria are seen in 

 the peripheral sarcoplasm 

 and around the nucleus at 

 the right. Meves' technic. 

 X 2000. 



cle the fiber has a greater diameter, is more nearly circular in cross sec- 

 tion, the myofibrils are scattered throughout its diameter, and the nuclei 

 are peripheral, lying just beneath the sarcolemma. The nuclei are envel- 



